In 3rd generation partnership project radio access network long term evolution (3GPP-LTE) (hereinafter, referred to as “LTE”), orthogonal frequency division multiple access (OFDMA) is adopted as a downlink communication scheme, and single carrier frequency division multiple access (SC-FDMA) is adopted as an uplink communication scheme (e.g., see Non-Patent Literatures 1, 2 and 3.)
In LTE, a radio communication base station apparatus (hereinafter, abbreviated as “base station”) communicates with a radio communication terminal apparatus (hereinafter, abbreviated as “terminal”) through allocation of resource blocks (RBs) in a system band to terminals, per time unit referred to as “subframe.” The base station also transmits, to the terminal, downlink control information (i.e., L1/L2 control information) to notify the result of resource allocation for downlink data and uplink data. This downlink control information is transmitted to terminals through downlink control channels, such as physical downlink control channel (PDCCH). Each PDCCH occupies a resource composed of one or more contiguous control channel elements (CCEs). In LTE, the number of CCEs (i.e., the number of connected CCEs) (hereinafter, referred to as “CCE aggregation level”) occupied by the PDCCH is set by selecting one of the numbers 1, 2, 4, and 8, depending on the number of information bits in downlink control information or the condition of propagation paths of the terminal, LTE supports a frequency band with a maximum width of 20 MHz as a system bandwidth.
Allocation control information transmitted from a base station is referred to as downlink control information (DCI). The base station transmits a plurality of pieces of DCI at the same time when allocating a plurality of terminals to a single subframe. In this case, the base station includes CRC bits, which are masked (or scrambled) with each ID of destination terminals, in each piece of DCI in order to identify each destination terminal of the DCI, and transmits the DCI. Then, the terminal performs blind decoding on a plurality of pieces of DCI, which may be addressed to the terminal by demasking (or descrambling) the CRC bits with a terminal ID of the terminal, and detects the DCI addressed to the terminal.
In addition, DCI includes modulation and channel coding scheme (MCS) and information on resource allocated to a terminal by a base station (i.e., resource allocation information), for example. DCI also has a plurality of formats, such as a format for uplink, a format for downlink multiple input multiple output (MIMO) transmission, a format for downlink non-contiguous band allocation, for example. Accordingly, a terminal is required to receive both of downlink allocation control information including a plurality of formats (i.e., allocation control information related to downlink) and uplink allocation control information including a single format (allocation control information related to uplink).
For example, in the downlink allocation control information, a plurality of different size formats is defined depending on, for example, a method of controlling a transmission antenna and a method of allocating resources of a base station. In the plurality of formats, a downlink allocation control information format (hereinafter, simply referred to as “downlink allocation control information”) used for performing band allocation of allocating RBs with continuous numbers (hereinafter, referred to as “contiguous band allocation”), has the same size as an uplink allocation control information format (hereinafter, simply referred to as “uplink allocation control information”) used for performing contiguous band allocation. These formats (i.e., DCI formats) include type information (e.g., a flag of one bit) indicating the type of allocation control information (i.e., downlink allocation control information or uplink allocation control information). Thus, the terminal can specify whether the DCI is downlink allocation control information or uplink allocation control information, through confirmation of the type information included in the allocation control information, even when the DCI indicating the downlink allocation control information has the same size as the DCI indicating uplink allocation control information.
The DCI format, of when the uplink allocation control information used for performing the contiguous band allocation is transmitted, is referred to as DCI format 0 (hereinafter, referred to as “DCI 0”), and the DCI format, of when the downlink allocation control information used for performing the contiguous band allocation is transmitted, is referred to as DCI format 1A (hereinafter, referred to as “DCI 1A”). As described above, DCI 0 and DCI 1A have the same size and can be distinguished according to the type information, and therefore DCI 0 and DCI 1A are expressed as “DCI 0/1A” together in the following explanation.
Other than the above described DCI formats, the downlink also involves: DCI format 1 (hereinafter, referred to as “DCI 1”) used for performing band allocation of allocating RBs with discontinuous numbers (hereinafter, referred to as “non-contiguous band allocation”); DCI formats 2 and 2A (hereinafter, referred to as “DCI 2, 2A”) used for allocating spatial multiplexing MIMO transmission; the format of downlink allocation control information (i.e., “beam forming allocation downlink format,” hereinafter referred to as “DCI format 1B”) used for allocating beam forming transmission; and the format of downlink allocation control information (i.e., “multiuser MIMO allocation downlink format,” hereinafter referred to as “DCI format 1D”) used for allocating multiuser MIMO transmission, for example. DCIs 1, 2, 2A, 1B, and 1D are formats used depending on downlink transmission modes of terminals (i.e., non-contiguous band allocation, spatial multiplexing MIMO transmission, beam forming transmission, and multiuser MIMO transmission) and set for each terminal. In contrast, DCI 0/1A does not depend on the transmission modes and can be used for a terminal in any transmission mode, i.e., can be used commonly used for all terminals. When DCI 0/1A is used, single-antenna transmission or transmit diversity is adopted as a default transmission mode. In contrast, studies are underway to use DCI format 0A used for performing non-contiguous band allocation and DCI format 0B used for allocating spatial multiplexing MIMO transmission, as formats for uplink allocation. Those formats are set for each terminal.
In addition, a method of limiting CCEs, which are targeted for blind decoding, for each terminal has been discussed for the purpose of reducing the number of blind decoding attempts in order to decrease the circuit scale of a terminal. This method limits a CCE region that may be a target of the blind decoding performed by each terminal (hereinafter, referred to as “search space”). A unit of the CCE region allocated to each terminal (i.e., equivalent to a unit targeted for the blind decoding) is referred to as “downlink control information allocation region candidate (i.e., DCI allocation region candidate)” or “decoding-target unit region candidate.”
In LTE, a search space is randomly set for each terminal. The number of CCEs forming the search space is defined per a CCE aggregation level of PDCCH. For example, the numbers of CCEs forming the search space are 6, 12, 8, and 16, corresponding to the CCE aggregation levels of PDCCHs 1, 2, 4, and 8, respectively. In this case, the numbers of decoding-target unit region candidates are six (6=6÷1), six (6=12÷2), two (2=8÷4), and two (2=16÷8), corresponding to the CCE aggregation levels of PDCCHs 1, 2, 4, and 8, respectively. In other words, the number of decoding-target unit region candidates is limited to sixteen in total. Consequently, the number of blind decoding attempts can be reduced since each terminal needs to perform the blind decoding on only a decoding-target unit region candidate group in the search space allocated to the terminal. In this case, the search space of each terminal is set using a terminal ID of each terminal and a hash function of performing randomization. This terminal-specific CCE region is referred to as “UE specific search space (UE-SS).”
Meanwhile, PDCCH also includes control information that is notified to a plurality of terminals at the same time and is used for data allocation common to all terminals (e.g., allocation information on downlink broadcast signals and allocation information on paging signals) (hereinafter, referred to as “control information for a common channel”). A CCE region common to all terminals that need to receive the downlink broadcast signals (hereinafter, referred to as “common search space (C-SS)”) is used for PDCCH, in order to transmit the control information for a common channel. In C-SS, only six decoding-target unit region candidates exist in total, i.e., four candidates (4=16÷4) and two candidates (2=16÷8), corresponding to CCE aggregation levels 4 and 8, respectively.
In UE-SS, a terminal performs blind decoding on two DCI formats having different sizes, i.e., DCI format commonly used for all terminals (DCI 0/1A) and DCI format depending on a transmission mode (one of DCIs 1, 2, and 2A). For example, in UE-SS, the terminal performs the blind decoding sixteen times as described above on each of the two DCI formats having different sizes. The transmission mode notified from a base station determines which two DCI formats having different sizes are processed through blind decoding. In C-SS, the terminal performs the blind decoding six times as described above on each of DCI 1A and DCI format 1C (i.e., twelve times of blind decoding in total), which is the format for common channel allocation (hereinafter, referred to as “DCI 1C”), regardless of the notified transmission mode. In other words, the terminal performs the blind decoding forty-four times in total on a subframe basis.
DCI 1A used for a common channel allocation has the same size as DCI 0/1A used for data allocation of an individual terminal, and they are distinguished by a terminal TD. Accordingly, even in C-SS, a base station can transmit DCI 0/1A used for terminal-specific data allocation without increasing the number of blind decoding attempts in the terminal.
In the meantime, the standardization of 3GPP LTE-Advanced (hereinafter, referred to as “LTE-A”), which achieves faster communication than LTE, has been started. In LTE-A, introducing a base station and a terminal (hereinafter, referred to as “LTE-A terminal”) that can communicate with each other through a wideband frequency of 40 MHz or more is expected in order to achieve the maximum downlink transmission speed of 1 Gbps or more and the maximum uplink transmission speed of 500 Mbps or more. An LTE-A system is required to accommodate not only LTE-A terminals but also terminals supporting an LTE system (hereinafter, referred to as “LTE terminals”).
Furthermore, in LTE-A, introducing a radio communication relay apparatus (hereinafter, referred to as “relay station” or “relay node (RN)”) is defined in order to achieve enhancement of coverage (see, FIG. 1). Accordingly, standardization of downlink control channel from a base station to a relay station (hereinafter, referred to as “R-PDCCH”) is advanced (e.g., see Non-Patent Literatures 4 to 7). The following matters about R-PDCCH are considered in the present stage. FIG. 2 shows an example R-PDCCH region.
(1) The position to start mapping in the time domain direction of R-PDCCH is fixed to the fourth OFDM symbol from the beginning of a single subframe. This position does not depend on the proportion of PDCCH in the time domain direction.
(2) Each R-PDCCH occupies a resource formed by a single or a plurality of contiguous relay-control channel elements (R-CCEs). The number of REs forming a single R-CCE differs depending on a slot or arrangement of reference signals. Specifically, in slot 0, R-CCE is defined as a resource region that has a range from the fourth OFDM symbol to the end of slot 0 in a time direction and has a width of 1 RB in a frequency direction (provided that regions on which reference signals are mapped are excluded). In slot 1, R-CCE is defined as a resource region that has a range from the start of slot 1 to the end of slot 1 in the time direction and has the width of 1 RB in the frequency direction (provided that regions on which reference signals are mapped are excluded). However, a proposal has also been made in which the above described resource region in slot 1 is divided into two parts and defining each part as a single R-CCE.